This is a repository copy of Experimental study on the supercritical startup of cryogenic loop heat pipes with redundancy design.

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1 This is a repository copy of Experimental study on the supercritical startup of cryogenic loop heat pipes with redundancy design. White Rose Research Online URL for this paper: Version: Accepted Version Article: Guo, Y, Lin, G, Bai, L orcid.org/ et al. (5 more authors) (2016) Experimental study on the supercritical startup of cryogenic loop heat pipes with redundancy design. Energy Conversion and Management, 118. pp ISSN This manuscript version is made available under the CC-BY-NC-ND 4.0 license Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher s website. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by ing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. eprints@whiterose.ac.uk

2 Experimental study on the supercritical startup of cryogenic loop heat pipes with redundancy design Yuandong Guo 1, Guiping Lin 1, Lizhan Bai 1,3,*, Xueqin Bu 1, Hongxing Zhang 2, Jiang He 2, Jianyin Miao 2, Dongsheng Wen 3 1 Laboratory of Fundamental Science on Ergonomics and Environmental Control, School of Aeronautic Science and Engineering, Beihang University, Beijing , PR China 2 Beijing Key Laboratory of Space Thermal Control Technology, Beijing Institute of Spacecraft System Engineering, China Academy of Space Technology, Beijing , PR China 3 School of Chemical and Process Engineering, University of Leeds, Leeds, LS2 9JT, UK Abstract: Cryogenic loop heat pipe (CLHP) is one of the key components in the future space infrared exploration system, which enables effective and efficient cryogenic heat transport over a long distance with a flexible thermal link. To realize reliable and long life operation, a CLHP-based thermal control system with redundancy design was proposed in this work, where two nitrogen-charged CLHPs were employed to provide cryocooling at K. This study focused on the supercritical startup of the CLHPs with redundancy design, and an extensive experimental study under four possible working modes was conducted. Experimental results showed that with 2.5W applied to the secondary evaporator, each CLHP could realize the supercritical startup successfully in the normal working mode; however, the required time differed a lot because the difference in the transport line diameter significantly affected the cryocooling capacity to the primary evaporator. In the backup conversion mode, instant switch of the two primary evaporators may cause an operation failure, and an auxiliary operation of the secondary evaporator in advance was necessary to make the primary liquid line filled with liquid. In the malfunction conversion mode, the simulated infrared detector had to be first shut down, but the time needed for the backup CLHP to realize the supercritical startup became obviously shorter than that in the normal working mode, because the primary evaporator of the backup CLHP was always in a cryogenic state. In the dual operation mode, the two CLHPs could realize the supercritical startup simultaneously, but a temperature oscillation phenomenon was observed, which can be eliminated by increasing the heat load applied to the secondary evaporator. Keywords: loop heat pipe; cryogenic; supercritical startup; working mode; experiment 28 * Corresponding author. Tel.: ; Fax: address: bailizhan@buaa.edu.cn (L. Bai). 1

3 29 1 Introduction 30 Heat pipe is an advanced two-phase heat transfer device, which combines the principles of both thermal 31 conduction and liquid/vapor phase change to efficiently manage the heat transfer processes between two solid 32 interfaces, and its operating principle, performance and characteristics were detailed in Refs.[1, 2]. Since the first 33 basic concept of heat pipe was proposed by Gaugler [3], it has attracted the interests of researchers worldwide, 34 and various types of heat pipes operating at different temperature ranges have been developed to satisfy specific 35 application requirements [4-7]. So far, heat pipes have been applied in a variety of areas in both space and 36 terrestrial surroundings such as in the building natural ventilation system, electronics cooling, industrial waste 37 heat recovery and spacecraft thermal control [8-13]. 38 As one of the latest developments of the heat pipe technology, cryogenic loop heat pipe (CLHP) can realize 39 efficient heat transport at the cryogenic temperatures. Depending on the working fluids charged, CLHPs can 40 operate in a specific low temperature range, i.e., propane for the operating temperature range of K, 41 oxygen for K, nitrogen for K, neon for K and hydrogen for K, and when helium is 42 selected as the working fluid, the operating temperature can be reached as low as 2-4 K. Compared with 43 traditional cryogenic heat pipes, CLHP exhibits a variety of advantages such as higher heat transfer capacity, 44 longer heat transport distance and stronger antigravity capability, as well as good flexibility for complex structures. 45 An important example for the CLHP applications is in the thermal control of the space infrared exploration system 46 where the infrared sensors/detectors have to be maintained at K or even lower temperatures [14, 15]. 47 For ambient loop heat pipe, the working fluids charged are typically ammonia, acetone, water, etc., which are in 48 the two-phase state at ambient temperature. Because the evaporator wick is always saturated by the liquid through 49 the matching design of the compensation chamber volume and the working fluid inventory, heat load can be 50 applied to the evaporator immediately to initiate the startup. However, for CLHP, the situation becomes much 2

4 51 different. Because the critical temperatures of the cryogenic working fluids are much lower than the ambient 52 temperature, i.e., the critical temperature of nitrogen is about 126 K, and the cryogenic working fluids are in the 53 supercritical state at ambient temperature. In order to start up the CLHP, prior to the application of heat load to the 54 evaporator, the working fluid inside must be first cooled to below its critical temperature until it condenses and 55 saturates the evaporator wick, and an auxiliary measure will be needed to help realize the liquid saturation of the 56 evaporator wick [16]. From the above analysis, a CLHP will inevitably experience a supercritical startup to realize 57 the normal operation. 58 So far, several types of CLHPs employing different auxiliary measures to realize the supercritical startup have 59 been developed, such as the gravity-assisted method [16], the secondary evaporator method [17-22], the auxiliary 60 loop method [23-28] and the capillary pump method [29-30], as reviewed in our previous paper [31]. Because the 61 CLHP employing the auxiliary loop method can realize the supercritical startup in space microgravity 62 environment, and can manage the parasitic heat load effectively with a relatively high heat transport capacity, it is 63 the most suitable for space applications and is selected as the object of study in this work. Below provides a brief 64 review on the development of this type of CLHP. 65 James et al. [23] proposed an ethane-charged CLHP operating at a temperature range of K for passive 66 optical bench cooling applications. Experimental results showed that the CLHP could reliably realize the 67 supercritical startup from an initially supercritical temperature of 335K with a heat load of 6 applied to the 68 secondary evaporator,. With a heat load of 5W applied to the secondary evaporator to manage the parasitic heat 69 load, the CLHP achieved a 5 heat transport capability at 215K. Hoang et al. [14] developed a nitrogen-charged 70 CLHP operating in a temperature range of K for across-gimbal cryocooling, which could successfully 71 realize the supercritical startup from an initially supercritical temperature of 298K, with a heat load of 5W applied 72 to the secondary evaporator. The maximum heat transport capability was determined to be 5W over a transport 3

5 73 distance of 4.3m. Later, a hydrogen-charged CLHP with a swing volume design was experimentally studied to 74 reduce the system volume/weight. The CLHP could realize the supercritical startup with a heat load of 2.5W 75 applied to the secondary evaporator when the shroud temperature was 200K; however, a supercritical startup 76 failure occurred in a 298K environment due to large parasitic heat load. The CLHP could operate at a temperature 77 range of 20-30K, and a maximum heat transport capability of 1 over a transport distance of 2.5m was obtained 78 with a 80K shroud [15]. To solve important problems in cryogenic integration, Bugby et al. [24, 25] developed 79 three CLHPs: an across-gimbal CLHP, a short and a long transport length miniaturized CLHP. The across-gimbal 80 CLHP employed nitrogen as the working fluid with a heat transport range of 2-2. Both the short and long 81 transport length CLHPs utilized neon as the working fluid and operated at a temperature range of 30-40K. 82 Preliminary test results showed that the short and long transport length CLHPs were able to transport a heat load 83 of W and W respectively. Gully et al. [26] designed and experimentally studied a nitrogen-charged 84 CLHP, where the effects of heat load, charged pressure and radiation heat load were studied in steady state 85 operation, and a maximum heat load of 19W was achieved over a transport distance of 0.5m. All the CLHPs 86 mentioned above employed a single-tube condenser where the flow resistance is relatively large. In order to 87 reduce the flow resistance in the condenser and increase the heat transport capacity of CLHPs, Zhao et al. [27, 28] 88 first proposed a nitrogen-charged CLHP with a parallel condenser design. Experimental results confirmed that the 89 CLHP could operate with a high heat transfer capacity up to 41W across a 0.48m transport distance. 90 From the above review, it is clear that CLHP is an effective and efficient cryogenic heat transport device with 91 many potential cryogenic applications. However, most current studies are still in the proof-of-concept stage, 92 where proper design of the evaporator and condenser structures to satisfy the connection requirements with the 93 heat source and heat sink has not been considered. At the same time, for space applications especially for the 94 space infrared exploration system, a miniaturization design with reduced system volume/weight should be 4

6 95 conducted. Besides these aspects, high reliability and long life design is of high priority, because the design life of 96 the next generation infrared observation satellites is over ten years or even longer, and on-orbit maintenance is 97 generally impossible for most space missions. Although there is no moving parts in the CLHPs, which promises a 98 long life nature, however, learning from the experience of applying ambient loop heat pipes in the spacecraft 99 thermal control system, it is possible that the operation anomaly or failure may occur for the loop heat pipes under 100 orbital conditions occasionally. For instance, for the spacecraft TacSat 4 launched in 2011, the Central Thermal 101 Bus design of the thermal control system utilized an ambient loop heat pipe to acquire/transport waste heat from 102 the on-board electronics and reject it to space via radiator panels. After a period of operation, operation anomaly 103 occurred for the loop heat pipe, resulted in much degraded performance of the thermal control system [32]. 104 Several years ago, operation anomaly also occurred for a loop heat pipe applied in the thermal control system of 105 the Geoscience Laser Altimetry System on the ICESat Satellite [33]. Therefore, it is quite necessary to adopt the 106 redundancy design to improve the operation reliability and realize a long life operation. 107 In order to address these issues and promote the future space applications of CLHPs, a CLHP-based thermal 108 control system with redundancy design was proposed in this work, where a miniature nitrogen-charged CLHP was 109 designed and fabricated considering appropriate interfaces with the heat source and heat sink, i.e., a saddle design 110 was used to realize easy connection between the cylindrical evaporator and simulated infrared detector, and a 111 novel cylindrical condenser design provided convenient interface with the cold finger of a pulse tube refrigerator. 112 To the best of our knowledge, this is the first experimental study on the CLHPs with redundancy design, and here 113 we mainly focus on the supercritical startup process, which is a very complicated but indispensable process prior 114 to the attainment of normal operation for CLHPs. Different working modes for the supercritical startup have been 115 defined due to the adoption of redundancy design, and extensive experimental studies were conducted, as 116 described in detail below. 5

7 117 2 Experimental setup 118 Fig. 1 shows the schematic view of the experimental rig in this work, which was comprised of two CLHPs, a 119 heating system, a cryogenic cooling system, a temperature measurement and data acquisition system and a 120 thermal-vacuum chamber. Fig. 2 shows the CLHP-based thermal control system with redundancy design where 121 the two CLHPs (#1 and #2) were connected with a pulse tube refrigerator respectively, and Fig. 3 shows the photo 122 of the CLHPs in the experiment. Because the CLHPs both employed an auxiliary loop to help realize the 123 supercritical startup, it was composed of a main loop and a secondary loop in addition to a gas reservoir. The main 124 loop included a primary evaporator, a primary CC (compensation chamber), a primary condenser and primary 125 vapor and liquid lines. The secondary loop consisted of a secondary evaporator, a secondary CC, a secondary 126 condenser and secondary loop lines. The gas reservoir with a comparatively large volume was utilized to reduce 127 the system pressure at ambient state, and it was connected to the inlet of the primary condenser. All the 128 components of the CLHP were made of stainless steel except that the primary and secondary evaporator wicks 129 were made of sintered nickel powders. The operating temperature of the CLHPs was designed in the range of K, and nitrogen whose purity was greater than % was charged as the working fluid. Table presents the basic parameters of the CLHPs, where CC represents the compensation chamber and OD and ID 132 represent the outer and inner diameters respectively. 133 To realize the redundancy design, the primary evaporators of the two CLHPs were coupled to a copper saddle, 134 as shown in Fig. 4, and a thin-film electric resistance heater was directly attached to the copper plate to simulate 135 the heat generated from a space infrared detector. The heat load of the electric heater can be adjusted from 0 to by altering the DC power output voltage with an uncertainty of ±5.0%, which can be transferred to both 137 primary evaporators through the copper saddle. In addition, the two secondary evaporators of the CLHPs were 138 both attached by a thin-film electric resistance heater respectively to provide auxiliary heat loads during the 6

8 139 supercritical startup. Two pulse tube refrigerators were employed as the cryogenic heat sink, whose cold fingers 140 were connected with the two cylindrical primary condensers of the CLHPs respectively. The pulse tube 141 refrigerator is a refrigeration device based on the Simon expansion principle to obtain the cooling effect through 142 charging and discharging high purity helium gas in the pulse tube. It was composed of a cold finger, a compressor 143 and a controller, with a cooling capacity greater than 4W at 70K. The pulse tube cold finger and the compressor 144 made up the main body, and the controller modulate the input power for its operation. In the operation, the 145 compressor can produce continuous sinusoidal pressure fluctuations, and the refrigerant gas experiences 146 endothermic expansion at the low temperature side and exothermic compression at the high temperature side, to 147 achieve the cooling effect through the phase modulation mechanism. 148 To reduce the parasitic heat load from the ambient surroundings, the CLHPs were placed in a thermal-vacuum 149 chamber, and all the components except the gas reservoir were covered with multilayer insulation materials. As 150 the pressure in the thermal-vacuum chamber can be maintained always below Pa, the parasitic heat load by 151 convective heat transfer becomes very small and can be safely neglected. Calibrated Type T thermocouples (TCs) 152 with measuring uncertainties of ±1.0 K were used to monitor the temperature variations of some characteristic 153 points along the loop, as shown in Fig. 5. To reduce the influence of gravity, the primary evaporator and the 154 primary condenser of the two CLHPs were always placed in a horizontal plane in the experiments Definition of different working modes 156 For a CLHP-based thermal control system with a redundancy design, it is expected that there are four possible 157 working modes: the normal working mode, the backup conversion mode, the malfunction conversion mode and 158 the dual operation mode, which covers the system operation in the whole lifetime. In the normal working mode, 159 only one CLHP with the connected pulse tube refrigerator is operating to provide cryocooling to the infrared 160 detector, while the other one is in the idle state. The backup conversion mode means to start up the CLHP in the 7

9 161 idle state and shut down the other one in operation to realize a smooth switch of the operation between the two 162 CLHPs, while maintaining the infrared detector at the normal working state. The malfunction conversion mode 163 means when the pulse tube refrigerator in operation breaks down or other similar issues emerge, it is necessary to 164 start up the other CLHP in the idle state to provide sufficient cryocooling. Under this condition, the infrared 165 detector in operation must be first shut down before the completion of the supercritical startup of the other CLHP. 166 At the end of the lifetime of the thermal control system, the cryocooling provided by a single one pulse tube 167 refrigerator may be insufficient due to the performance degradation after several years of operation. Under such a 168 condition, both CLHPs with their corresponding pulse tube refrigerators will be needed to operate simultaneously, 169 so as to provide sufficient cryocooling, which is termed as the dual operation mode. This working mode is of great 170 importance to guarantee the normal operation of the space infrared exploration system at the end of lifetime Experimental results and analysis Normal working mode 173 Fig. 6(a) shows the temperature variations of some characteristic points along the loop during the supercritical 174 startup of the #1 CLHP. As shown in Fig. 6(a), the supercritical startup was initiated from an initially supercritical 175 temperature of about 300K. At 22 min, the cold finger of the pulse tube refrigerator began to take effect, and the 176 temperatures at the secondary evaporator (TC10) and at the outlet of the primary condenser (TC7) dropped rapidly, 177 due to direct contact with the cold finger of the refrigerator. At 50 min, the temperatures at the secondary 178 evaporator, secondary evaporator outlet and primary condenser outlet dropped at a much faster rate, indicating 179 that the temperatures of nitrogen gas in the primary condenser and secondary CC dropped to their saturation 180 temperatures corresponding to the local pressure, and fluid condensation occurred there afterwards, making the 181 heat transfer proceed in a more efficient way. Several minutes later, the temperatures at the secondary evaporator 182 (TC10) and at the primary condenser outlet (TC7) began to drop slowly and remained at about 90 K, indicating 8

10 183 that nitrogen in the two-phase state began to exist there. As time went by, the inventory of liquid nitrogen in the 184 secondary evaporator would increase gradually, and it would fully saturate the secondary evaporator wick 185 eventually. At 60 min, a very large temperature drop occurred at the primary CC inlet (TC1) due to the arrival of 186 the condensate from the primary condenser, from which it can be inferred that when the secondary CC was cooled, 187 the temperature/pressure of the working fluid inside it dropped, and a fluid circulation from the gas reservoir 188 through the primary condenser, primary liquid line and the secondary loop line to the secondary CC should exist, 189 to continuously supply working fluid to the secondary CC. 190 With the temperature of the secondary evaporator remaining at 90 K for about 12 min, a heat load of 2.5W was 191 applied to the secondary evaporator (Q se (1) ), and no obvious temperature rise of the secondary evaporator (TC10) 192 was observed, indicating that the secondary evaporator was started up successfully. Under such a condition, the 193 secondary evaporator could act as a pump to drive the condensed liquid nitrogen in the primary condenser to the 194 primary evaporator core and CC through the primary liquid line (path 1), as evidenced by continous temperature 195 drop at the outlet of the primary CC (TC8), at the primary evaporator (TC3) and at the primary evaporator outlet 196 (TC4). Note that, there was another flow path from the secondary evaporator to the primary evaporator core, i.e. 197 from the secondary evaporator through the primary vapor line, primary evaporator vapor grooves and primary 198 evaporator wick to the primary evaporator core (path 2). Because of the difference in flow resistance between the 199 two paths from the secondary evaporator to the primary evaporator core, more than 80% of the heat load on the 200 secondary evaporator would flow through the path 1. As the primary evaporator wick began to be saturated with 201 liquid coming from the primary condenser, the capillary pressure developed by the primary evaporator wick 202 would totally shut down path 2 and force all the heat load on the secondary evaporator to flow through the path 1, 203 resulted in a more efficient cooling to the primary evaporator. This was evidenced by more rapid cooldown rate of 204 the primary evaporator and a sudden temperature drop at the primary evaporator outlet (TC4) at about 150 min. 9

11 205 With the operation of the secondary evaporator for about 90 min, the primary evaporator temperature (TC3) 206 dropped to about 90K and began to remain unchanged, indicating that nitrogen in the two-phase state began to 207 exist in the primary evaporator wick. As time went by, the inventory of liquid nitrogen in the primary evaporator 208 would increase gradually and fully saturate the primary evaporator wick eventually. With the primary evaporator 209 temperature remaining at 90K for several minutes, a heat load of 1W was applied to the copper saddle (Q cs ), and 210 the heat load applied to the secondary evaporator (Q se (1) ) was reduced to 1W. No obvious temperature rise of the 211 primary evaporator was observed, indicating that the primary evaporator was started up successfully. With the 212 normal operation of the primary evaporator, the #1 CLHP realized the supercritical startup. 213 Fig. 6 (b) shows the temperature variations of some characteristic points along the loop during the supercritical 214 startup of the #2 CLHP, which were similar to those of #1 CLHP during the whole supercritical startup process. 215 However, in Fig. 6(b), the secondary evaporator had to operate for about 240 min with a heat load of 2.5W (Q se (2) ) 216 to realize the large temperature drop of the primary evaporator from the ambient state (300K) to the saturation 217 temperature (90K), which became much longer than that of #1 CLHP (90 min). This phenomenon can be 218 explained according to the difference in the structural size between the two CLHPs: because the inner diameter of 219 the liquid line of #2 CLHP (1.0mm) is much smaller than that of #1 CLHP (2.0mm), the flow resistance in the 220 liquid line of #2 CLHP becomes much larger. As discussed above, there are two paths from the secondary 221 evaporator to the primary evaporator core, and only for path 1, the condensate in the primary condenser can be 222 driven into the primary evaporator core with a much larger cryocooling capacity; at the same time, the 223 cryocooling capacity is strongly dependent on the mass flowrate of the working fluid in path 1. For #2 CLHP, the 224 considerably increased flow resistance in the primary liquid line changes the balance of the flow resistance 225 between the two paths, which leads to much reduced mass flowrate in path 1, compared to the situation in #1 226 CLHP. Therefore a careful flow resistance analysis should be conducted in the CLHP design to effectively 10

12 227 accelerate the supercritical startup process Backup conversion mode 229 In the backup conversion mode, a smooth switch of the operation of the two primary evaporators will be 230 completed while the infrared detector always keeps working, and the two pulse tube refrigerators have to be 231 operating simultaneously for a period of time. Because the coefficient of performance (COP) of the pulse tube 232 refrigerator is only 2.0% or even lower when it is operating at a cryogenic temperature range of K, it is a 233 high energy-consuming component in the thermal control system. As the space power supply is limited, it is very 234 important to accelerate the backup conversion by reducing the time of simultaneous operation of the two pulse 235 tube refrigerators for the energy saving purpose Backup conversion without auxiliary operation 237 In order to reduce the backup conversion time maximally, backup conversion from #1 to #2 CLHP without an 238 auxiliary operation of the secondary evaporator in advance was first experimentally studied, as shown in Fig. 7, 239 where an operation failure occurred. At the initial state, the #1 CLHP was in normal operation with a heat load 240 of 4W and 0.5W applied to the copper saddle and secondary evaporator respectively. At about 15 min, #2 pulse 241 tube refrigerator was started, and the temperatures of the primary condenser (TC17) and secondary evaporator 242 (TC21) of the #2 CLHP began to drop gradually. At about 70 min, the temperature of the secondary evaporator 243 (TC21) of the #2 CLHP had dropped to the saturation temperature (90K) and remained at this value for several 244 minutes, indicating that nitrogen in the two-phase state began to exist there and liquid nitrogen would saturate the 245 secondary evaporator wick eventually. Then the #1 pulse tube refrigerator and heat load applied to the secondary 246 evaporator of the #1 CLHP was turned off, and a heat load of 0.5W was applied to the secondary evaporator of the 247 #2 CLHP simultaneously, trying to realize an instant switch of the operation of the two primary evaporators. The 248 primary evaporator temperature of the #2 CLHP (TC14) began to rise continuously, indicating that an operation 11

13 249 failure occurred. 250 The reason for the operation failure was analyzed below. After the #1 CLHP with the connected #1 pulse tube 251 refrigerator was totally shut down, a heat load of 4W and 0.5W was actually applied to the primary and secondary 252 evaporators of the #2 CLHP respectively. Although the primary and secondary evaporator wicks of the #2 CLHP 253 had been saturated by the liquid nitrogen, the working fluid in the primary liquid line was still in the vapor state 254 with a relatively high temperature. With the circulation of the working fluid in the primary loop of the #2 CLHP, 255 the nitrogen vapor in the primary liquid line was pushed into the primary evaporator core, thus causing 256 insufficient liquid supply to the primary evaporator wick, and the normal operation would be interrupted 257 accompanied by a continuous temperature rise of the primary evaporator of the #2 CLHP Backup conversion with auxiliary operation 259 In order to address the above issue and ensure a successful backup conversion, in the subsequent experiment, an 260 auxiliary operation of the secondary evaporator of the #2 CLHP with a heat load of 2.5W was first implemented 261 before the total shut down of the #1 CLHP with the connected #1 pulse tube refrigerator, to guarantee that the 262 primary liquid line of the #2 CLHP was filled with liquid nitrogen prior to the startup of the primary evaporator. 263 As shown in Fig. 8, after the secondary evaporator wick of the #2 CLHP was fully saturated by the liquid 264 nitrogen, different from the case in Fig. 7, a heat load of 2.5W was first applied to the secondary evaporator to 265 start up the secondary loop, before the total shut down of the #1 CLHP with the #1 pulse tube refrigerator, With 266 the circulation of the working fluid in the secondary loop of the #2 CLHP for about 10 minutes, the primary liquid 267 line would be filled with liquid nitrogen, under such a condition, it was ready to start up the primary loop of the #2 268 CLHP. At about 102 min, the #1 pulse tube refrigerator and heat load applied to the secondary evaporator of the 269 #1 CLHP was turned off, and a reduced heat load of 0.5W was applied to the secondary evaporator of the #2 270 CLHP simultaneously, to realize a switch of the operation of the two primary evaporators. It was as expected that 12

14 271 no obvious temperature rise of the primary evaporator of the #2 CLHP (TC14) was observed, indicating that a 272 successful backup conversion was achieved. 273 In the backup conversion mode, because the primary evaporator of the backup CLHP has already been filled 274 with liquid nitrogen, the purpose for the auxiliary operation of the secondary evaporator is only to make the 275 primary liquid line flooded by the liquid nitrogen, and the time needed for the auxiliary operation of the secondary 276 evaporator is remarkably reduced compared with the case in the normal working mode Malfunction conversion mode 278 Fig. 9 shows the temperature variations of some characteristic points along the loop during the supercritical 279 startup in the malfunction conversion mode. At the initial state, the #2 CLHP was in the normal operation with a 280 heat load of 4W and 0.5W applied to the copper saddle and secondary evaporator respectively. At 25 min, #2 pulse 281 tube refrigerator was shut down to simulate the operation failure in real applications. Several minutes later, a 282 sudden temperature drop occurred at the outlet of the primary evaporator of the #1 CLHP, indicating that a certain 283 amount of heat load applied to the copper saddle was transferred to the primary evaporator of the #1 CLHP where 284 vapor was generated and flowed out of the primary evaporator. At 30 min, it was found that the primary 285 evaporator temperature of the #2 CLHP (TC14) rose quickly, indicating that an operation failure occurred for the 286 thermal control system, then the heat load applied to the copper saddle was turned off, and the #1 pulse tube 287 refrigerator was started to initiate a malfunction conversion. 288 As shown in Fig. 9, the supercritical startup process of the #1 CLHP is similar to that in Fig. 6(a); however, the 289 time needed for the auxiliary operation of the secondary evaporator was reduced remarkably, i.e., in Fig. 6(a), the 290 secondary evaporator had to operate for about 90 minutes to realize the cryocooling to the primary evaporator, 291 while here only 40 minutes was needed, that was because the primary evaporator of the backup CLHP (#1 CLHP) 292 was always in a cryogenic state, and the cryocooling needed to realize the liquid saturation of the primary 13

15 293 evaporator wick decreased considerably. In the malfunction conversion mode, the simulated infrared detector had 294 to be shut down for about 120 minutes Dual operation mode 296 Fig. 10 shows the temperature variations of some characteristic points along the loop during the supercritical 297 startup of the two CLHPs. As shown in Fig. 10, at the initial state, all the characteristic points along the loop were 298 at the ambient temperature of about 295 K, and the supercritical startup of the two CLHPs was initiated 299 simultaneously. After the secondary evaporator wicks of the two CLHPs were fully saturated by the liquid, a heat 300 load of 2.5W was applied to the secondary evaporators of the two CLHPs respectively to start up the secondary 301 loop. After the primary evaporator wicks of the two CLHPs were fully saturated by the liquid, a heat load of 4W 302 was applied to the copper saddle, and the heat loads applied to the secondary evaporators were both reduced to 1W. 303 No obvious temperature rise of each primary evaporator was observed, indicating that the normal operation for 304 each CLHP was established, and successful supercritical startup of the two CLHPs in dual operation mode was 305 achieved. During the supercritical startup, it took about 120 minutes for the auxiliary operation of the secondary 306 evaporators to realize the cryocooling to the two primary evaporators from an ambient temperature (295K) to their 307 saturation temperature (90K), which was between the times required by #1 and #2 CLHPs in the normal working 308 mode, i.e., 90 and 240 minutes respectively, indicating that heat transfer between the #1 and #2 primary 309 evaporators through the copper block should exist to help the #2 primary evaporator to accelerate the temperature 310 drop process. 311 In the dual operation mode, obvious temperature oscillation phenomenon was observed for the #1CLHP when 312 the heat load applied to the secondary evaporator was relatively small. As shown in Fig. 11, when the heat load 313 applied to the secondary evaporator of the #1CLHP was 1W, obvious temperature oscillation at the inlets of the 314 primary CC (TC1) and the primary condenser (TC5) was observed; however, when the heat load applied to the 14

16 315 secondary evaporator was increased to 1.5W, the temperature oscillation phenomenon would disappear completely. 316 This phenomenon may be explained as follows: when the heat load applied to the secondary evaporator is 317 relatively small, the total mass flowrate in the primary liquid line was small accordingly, and the return subcooling 318 may be insufficient to offset the heat leak to the primary CC from both the primary evaporator and the ambient 319 surroundings, causing an increase of the temperature/pressure in the primary CC and resultant oscillation 320 movement of the working fluid along the main loop; however, when the heat load applied to the secondary 321 evaporator is increased to appropriate relatively high level, the total mass flowrate in the primary liquid line 322 increases accordingly, and the return subcooling becomes sufficient to offset the heat leak to the primary CC, 323 under this condition, oscillation flow in the main loop will be completely impossible Conclusions 325 A CLHP-based thermal control system with redundancy design for future space infrared exploration system was 326 designed and fabricated, and an extensive experimental study on the supercritical startup characteristics under four 327 possible working modes was conducted, where the temperature variations of some characteristic points along the 328 loop were presented, and the mechanisms responsible for specific phenomenon were analyzed. According to the 329 experimental results, important conclusions have been reached as summarized as follows: 330 In the normal working mode, with 2.5W applied to the secondary evaporator, each CLHP could successfully 331 realize the supercritical startup independently; however, the required time differed a lot because the 332 difference in the transport line diameter significantly affected the cryocooling capacity to the primary 333 evaporator, and a smaller diameter transport line may result in a much longer supercritical startup time. 334 In the backup conversion mode, instant switch of the two primary evaporators may cause an operation failure, 335 and an auxiliary operation of the secondary evaporator of the backup CLHP in advance was necessary to 336 make the primary liquid line filled with liquid and ensure a successful backup conversion. 15

17 337 In the malfunction conversion mode, the simulated infrared detector had to be first shut down, but the time 338 needed for the backup CLHP to realize the supercritical startup become obviously shorter than that in the 339 normal working mode because the primary evaporator of the backup CLHP was always in a cryogenic state, 340 and the cryocooling needed to realize the liquid saturation of the primary evaporator wick decreased 341 considerably. 342 In the dual operation mode, the two CLHPs could realize the supercritical startup simultaneously, and the 343 time needed was between the ones required by the two CLHPs in the normal working mode, due to the heat 344 transfer between the two primary evaporators through the copper saddle; however, a temperature oscillation 345 phenomenon was observed, which can be eliminated effectively by increasing the heat loads applied to the 346 secondary evaporator. 347 As the first reported experimental study on a CLHP-based thermal control system with redundancy design, the 348 experimental results and analysis provided here contribute greatly to the design and future space applications of 349 such an advanced thermal control system with high reliability and long lifespan. 350 Acknowledgement 351 This work was supported by the National Natural Science Foundation of China (No and ), 352 Beijing Natural Science Foundation (No ), and the EU Marie Curie Actions-International Incoming 353 Fellowships (FP7-PEOPLE-2013-IIF ). The authors are very grateful to Li Guoguang and Li Lei in the 354 Beijing Institute of Spacecraft System Engineering, China Academy of Space Technology, for their help in the 355 preparation of the experiments

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21 423 Table captions: 424 Table1 Basic parameters of the tested CLHPs

22 431 Figure captions: 432 Fig. 1 A schematic view of the experimental system 433 Fig. 2 A schematic view of the CLHP-based thermal control system with redundancy design 434 Fig. 3 Photo of the two CLHPs in the experiment 435 Fig. 4 Detailed structure of the two primary evaporators coupled by a copper saddle 436 Fig. 5 Thermocouple locations along the loop of the two CLHPs 437 Fig. 6 Supercritical startup process of the two CLHPs in the normal working mode 438 Fig. 7 Supercritical startup process without auxiliary operation in the backup conversion mode 439 Fig. 8 Supercritical startup process with auxiliary operation in the backup conversion mode 440 Fig. 9 Supercritical startup process in the malfunction conversion mode 441 Fig. 10 Supercritical startup process in the dual operation mode 442 Fig. 11 Temperature oscillation phenomenon in the dual operation mode

23 Table1 Basic parameters of the tested CLHPs Components Dimensions #1 CLHP #2CLHP Primary evaporator Casing OD/ID length of /mm Wick OD/ID length/mm Secondary evaporator Casing OD/ID length/mm Wick OD/ID length/mm Liquid line OD/ID length/mm Primary loop Condenser line OD/ID length/mm Vapor line OD/ID length/mm Liquid line OD/ID length/mm Secondaryloop Condenser line OD/ID length/mm Vapor line OD/ID length/mm Maximum capillary radius/ m Wick Porosity 55% 55% Permeability/m 2 > > Gas reservoir Volume/ml

24 Auxiliary DC power - + Vacuum pump Primary DC power Film electric resistance heater Secondary condenser Primary condenser - + Copper saddle Secondary evaporator Electric heater Pulse tube refrigerator Primary evaporator Thermal-vacuum chamber Primary condenser Secondary condenser Pulse tube refrigerator Auxiliary DC power Temperature measurement & data acquisition system 465 Fig. 1 A schematic view of the experimental system

25 #1 refrigerator #2 refrigerator #1 secondary evaporator #1 primary evaporator #2 secondary evaporator copper saddle #2 primary evaporator Fig. 2 A schematic view of the CLHP-based thermal control system with redundancy design #1 condenser #1 secondary evaporator and secondary CC #1 gas reservoir #1 primary evaporator and primary evaporator CC and evaporator and #2 condenser #2 secondary evaporator and secondary CC #2 gas reservoir #2 primary evaporator and primary CC Fig. 3 Photo of the two CLHPs in the experiment

26 Fig. 4 Detailed structure of the two primary evaporators coupled by a copper saddle

27 (a) #1 cryogenic loop heat pipe (b) #2 cryogenic loop heat pipe Fig. 5 Thermocouple locations along the loop of the two CLHPs

28 #1CLHP 2.5W 1W 1W Qcs Qse (1) 240 TC4 Temperature/K TC11 TC7 TC10 TC1 TC8 TC Time/min (a) #1 cryogenic loop heat pipe 4W #2CLHP 2.5W 0.8W TC15 Qcs Qse (2) 240 TC14 Temperature/K TC21 TC20 TC Time/min (b) #2 cryogenic loop heat pipe Fig.6 Supercritical startup process of the two CLHPs in the normal working mode

29 W #1CLHP 0.5W #2CLHP 300 OFF 0.5W Temperature/K TC14 TC10 TC19 TC3 TC21 TC17 (a) the whole range TC17 TC19 Qcs Qse (1) Qse (2) TC Time/min 4W #1CLHP 0.5W #2CLHP W TC19 Qcs Qse (1) Qse (2) Temperature/K TC14 TC17 TC3 TC21 TC19 TC TC10 TC Time/min (b) local enlargement Fig. 7 Supercritical startup process without auxiliary operation in the backup conversion mode

30 W #1CLHP 0.5W #2CLHP 300 OFF 2.5W 0.5W Temperature/K TC19 TC21 TC20 TC9 Qcs Qse (1) Qse (2) TC9 TC3 TC Time/min (a) the whole range 4W #1CLHP #2CLHP W 2.5W 0.5W Temperature/K TC9 TC20 TC21 TC14 TC9 Qcs Qse (1) Qse (2) 70 TC20 TC Time/min (b) local enlargement Fig. 8 Supercritical startup process with auxiliary operation in the backup conversion mode

31 W 4W #1CLHP OFF 2.5W 1W #2CLHP W OFF Qcs TC11 TC17 Qse (1) Qse (2) Temperature/K TC4 TC7 TC10 TC TC3 TC14 TC14 TC Time/min Fig. 9 Supercritical startup process in the malfunction conversion mode 4W #1CLHP 2.5W 1W #2CLHP 2.5W 1W Qcs Qse (1) Qse (2) Temperature/K TC21 TC10 TC22 TC3 TC12 TC11 TC4 TC Time/min Fig. 10 Supercritical startup process in the dual operation mode